[Java] Templates to support google-api-client library (#6838)

* Add support for the google-api-client

* When there's no return type, just return (no parsing the response)

* Update the deserialization to use a Jackson typeref.

* Delete the auth mustache files as we don't need them

* Stop generating unneeded auth files; get SBT/gradle working with the generated module

* Add builder-style methods to the google-api-client ApiClient for getting API instances

* Update the README to reflect the new client library option

* Generated overloaded methods to send query params as a Map<String, Object>

* Add files for Sample codes

* Add type in angle brackets to support Java 6

* Required query params must be added to the Map of params. Update sample code.

* Use explicit types instead of diamonds to support Java6 in one more place.

* Clean up javadoc warnings in generated code / remove pointless return statements

* Clean up extra newlines in generated code

samples/client/petstore/java/google-api-client/docs/EnumTest.md

@@ -0,0 +1,38 @@
+
+# EnumTest
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**enumString** | [**EnumStringEnum**](#EnumStringEnum) |  |  [optional]
+**enumInteger** | [**EnumIntegerEnum**](#EnumIntegerEnum) |  |  [optional]
+**enumNumber** | [**EnumNumberEnum**](#EnumNumberEnum) |  |  [optional]
+**outerEnum** | [**OuterEnum**](OuterEnum.md) |  |  [optional]
+
+
+<a name="EnumStringEnum"></a>
+## Enum: EnumStringEnum
+Name | Value
+---- | -----
+UPPER | &quot;UPPER&quot;
+LOWER | &quot;lower&quot;
+EMPTY | &quot;&quot;
+
+
+<a name="EnumIntegerEnum"></a>
+## Enum: EnumIntegerEnum
+Name | Value
+---- | -----
+NUMBER_1 | 1
+NUMBER_MINUS_1 | -1
+
+
+<a name="EnumNumberEnum"></a>
+## Enum: EnumNumberEnum
+Name | Value
+---- | -----
+NUMBER_1_DOT_1 | 1.1
+NUMBER_MINUS_1_DOT_2 | -1.2
+
+
+